* Stefan Hajnoczi (stefa...@redhat.com) wrote:
> Introduce lo_dirp_put() so that FUSE_RELEASEDIR does not cause
> use-after-free races with other threads that are accessing lo_dirp.
>
> Also make lo_releasedir() atomic to prevent FUSE_RELEASEDIR racing with
> itself. This prevents double-frees.
>
> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
> ---
> contrib/virtiofsd/passthrough_ll.c | 42 +++++++++++++++++++++++++-----
> 1 file changed, 36 insertions(+), 6 deletions(-)
>
> diff --git a/contrib/virtiofsd/passthrough_ll.c
> b/contrib/virtiofsd/passthrough_ll.c
> index ad3abdd532..f74e7d2d21 100644
> --- a/contrib/virtiofsd/passthrough_ll.c
> +++ b/contrib/virtiofsd/passthrough_ll.c
> @@ -1293,11 +1293,28 @@ static void lo_readlink(fuse_req_t req, fuse_ino_t
> ino)
> }
>
> struct lo_dirp {
> + gint refcount;
> DIR *dp;
> struct dirent *entry;
> off_t offset;
> };
>
> +static void lo_dirp_put(struct lo_dirp **dp)
> +{
> + struct lo_dirp *d = *dp;
> +
> + if (!d) {
> + return;
> + }
> + *dp = NULL;
> +
> + if (g_atomic_int_dec_and_test(&d->refcount)) {
> + closedir(d->dp);
> + free(d);
> + }
> +}
> +
> +/* Call lo_dirp_put() on the return value when no longer needed */
> static struct lo_dirp *lo_dirp(fuse_req_t req, struct fuse_file_info *fi)
> {
> struct lo_data *lo = lo_data(req);
> @@ -1305,6 +1322,9 @@ static struct lo_dirp *lo_dirp(fuse_req_t req, struct
> fuse_file_info *fi)
>
> pthread_mutex_lock(&lo->mutex);
> elem = lo_map_get(&lo->dirp_map, fi->fh);
> + if (elem) {
> + g_atomic_int_inc(&elem->dirp->refcount);
I don't understand what protects against reading the elem->dirp
here at the same time it's free'd by lo_releasedir's call to lo_dirp_put
> + }
> pthread_mutex_unlock(&lo->mutex);
> if (!elem)
> return NULL;
> @@ -1335,6 +1355,8 @@ static void lo_opendir(fuse_req_t req, fuse_ino_t ino,
> struct fuse_file_info *fi
> d->offset = 0;
> d->entry = NULL;
>
> + g_atomic_int_set(&d->refcount, 1); /* paired with lo_releasedir() */
> +
> fh = lo_add_dirp_mapping(req, d);
> if (fh == -1)
> goto out_err;
> @@ -1363,7 +1385,7 @@ static void lo_do_readdir(fuse_req_t req, fuse_ino_t
> ino, size_t size,
> off_t offset, struct fuse_file_info *fi, int plus)
> {
> struct lo_data *lo = lo_data(req);
> - struct lo_dirp *d;
> + struct lo_dirp *d = NULL;
> struct lo_inode *dinode;
> char *buf = NULL;
> char *p;
> @@ -1451,6 +1473,8 @@ static void lo_do_readdir(fuse_req_t req, fuse_ino_t
> ino, size_t size,
>
> err = 0;
> error:
> + lo_dirp_put(&d);
> +
> // If there's an error, we can only signal it if we haven't stored
> // any entries yet - otherwise we'd end up with wrong lookup
> // counts for the entries that are already in the buffer. So we
> @@ -1477,22 +1501,25 @@ static void lo_readdirplus(fuse_req_t req, fuse_ino_t
> ino, size_t size,
> static void lo_releasedir(fuse_req_t req, fuse_ino_t ino, struct
> fuse_file_info *fi)
> {
> struct lo_data *lo = lo_data(req);
> + struct lo_map_elem *elem;
> struct lo_dirp *d;
>
> (void) ino;
>
> - d = lo_dirp(req, fi);
> - if (!d) {
> + pthread_mutex_lock(&lo->mutex);
> + elem = lo_map_get(&lo->dirp_map, fi->fh);
> + if (!elem) {
> + pthread_mutex_unlock(&lo->mutex);
> fuse_reply_err(req, EBADF);
> return;
> }
>
> - pthread_mutex_lock(&lo->mutex);
> + d = elem->dirp;
> lo_map_remove(&lo->dirp_map, fi->fh);
> pthread_mutex_unlock(&lo->mutex);
>
> - closedir(d->dp);
> - free(d);
> + lo_dirp_put(&d); /* paired with lo_opendir() */
Is the &d really what's intended? That's the local stack variable, so
lo_dirp_put will set that local to NULL rather than the elem->dirp wont
it?
Dave
> +
> fuse_reply_err(req, 0);
> }
>
> @@ -1701,6 +1728,9 @@ static void lo_fsyncdir(fuse_req_t req, fuse_ino_t ino,
> int datasync,
> res = fdatasync(fd);
> else
> res = fsync(fd);
> +
> + lo_dirp_put(&d);
> +
> fuse_reply_err(req, res == -1 ? errno : 0);
> }
>
> --
> 2.21.0
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
